1. Field of the Invention
The present invention is directed to wireless communication systems and, more specifically, to providing channel state information feedback in wireless communication systems.
2. Description of the Art
A communication system may include transmitting and/or receiving DownLink (DL) signals, which are signals transmitted from Base Stations (BSs), such as NodeBs, to User Equipments (UEs), and UpLink (UL) signals, which are signals from transmitted from UEs to NodeBs. A UE, which may be a terminal, a mobile station, a Personal Computer (PC), or any other similar and/or suitable electronic device, may be fixed or mobile and may be a wireless device. A NodeB may be a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or any other similar and/or suitable device name for describing a device receiving UL signals from a UE.
A NodeB transmits data information to a UE using a Physical DL Shared CHannel (PDSCH) and transmits DL Control Information (DCI) to a UE using a Physical DL Control CHannel (PDCCH). A UE transmits data information to a NodeB using a Physical UL Shared CHannel (PUSCH) and transmits UL Control Information (UCI) to a NodeB using a Physical UL Control CHannel (PUCCH). If a UE transmits data information and UCI at a same Transmission Time Interval (TTI), the UE may multiplex UCI with data information in a PUSCH so as to not transmit UCI in a PUCCH.
UCI may include Channel State Information (CSI), which may include Channel Quality Indicator (CQI) information and Precoding Matrix Indicator (PMI) information. The CSI informs a NodeB of DL channel conditions that a UE experiences, and accordingly, the NodeB may select appropriate parameters, such as a transmission power, a Modulation and Coding Scheme (MCS), and any other similar and/or suitable parameters, for a PDSCH or PDCCH transmission to the UE and may ensure a desired BLock Error Rate (BLER) for transmitting respective data information or DCI. CQI provides a measure of a Signal to Interference and Noise Ratio (SINR) over sub-bands or over an entire operating DL BandWidth (BW), and the CQI may be provided as information indicating a highest MCS for which a BLER target for a data Transport Block Size (TBS) transmitted to the UE may be achieved. The PMI informs a NodeB how to combine a signal that is to be transmitted from multiple NodeB antennas to a UE in accordance with a Multiple-Input Multiple-Output (MIMO) method.
A UE measures CSI based on a DL Reference Signal (RS) transmitted from a NodeB. Different CSI processes may be associated with different CSI measurements that may be respectively obtained from different RSs or from a same RS at different parts of a DL BW. The CSI may be transmitted in a PUCCH or in a PUSCH. The CSI transmission in a PUCCH may be semi-statically configured by a NodeB so as to occur periodically, i.e., the CSI transmission in the PUCCH is a periodic CSI, and may support only small CSI payloads, such as CSI payloads having up to about 10 information bits, in order to avoid excessive overhead. The CSI in a PUSCH may be dynamically triggered by a NodeB, i.e., the CSI transmission in PUSCH is an aperiodic CSI, through a “CSI request” field in a DCI format conveyed by a PDCCH scheduling the PUSCH transmission and it may support large CSI payloads in order to provide the NodeB with detailed information for PDSCH scheduling.
FIG. 1 illustrates a PUSCH transmission structure in an UL TTI according to the related art.
Referring to FIG. 1, a UL TTI 100 may include one subframe 110, which includes two slots 120. Each slot 120 includes a number, NsymbUL, of symbols 130 for transmitting a UL RS transmission 140, data information, UCI and any other similar and/or suitable data or information. As shown in FIG. 1, each slot 120 includes 7 symbols 130 such that NsymbUL=7. The UL RS transmission 140 provides channel estimation and enables coherent demodulation of data information or UCI. The UL RS may be transmitted using a Zadoff-Zhu (ZC) sequence that is assigned a Cyclic Shift (CS) and the two UL RSs in a subframe may be further modulated by an Orthogonal Covering Code (OCC) having a value of {1 1} or {1 −1}. A PUSCH transmission BW includes frequency resource units which will be referred to as Resource Blocks (RBs). Each RB includes NscRB sub-carriers, or Resource Elements (REs), and a UE may be allocated MPUSCH RBs 150 for a PUSCH transmission such that a total of MscPUSCH=MPUSCH·NscRB REs.
FIG. 2 illustrates a UE transmitter block diagram for a PUSCH according to the related art.
Referring to FIG. 2, in the UE transmitter 200, coded CSI bits 205 and coded data bits 210 are multiplexed by multiplexer 220. A Discrete Fourier Transform (DFT) of combined data bits and CSI bits is then obtained by a DFT unit 230. Next, REs are provided to a sub-carrier mapping unit 240 so that REs corresponding to an assigned transmission BW are selected by a controller for transmission BW 250, and then Inverse Fast Fourier Transform (IFFT) is performed by an IFFT unit 260. Next, a Cyclic Prefix (CP) is inserted by a CP insertion unit 270 and filtering is performed by a time windowing unit 280 in order to generate a transmitted signal 290. An encoding process for data bits or CSI bits and a modulation process for all transmitted bits are omitted for brevity.
FIG. 3 illustrates a NodeB receiver block diagram for a PUSCH according to the related art.
Referring to FIG. 3, the NodeB receiver 300 receives a signal 310 and filters the received signal 310 using a time windowing unit 320. Next, a CP removal unit 330 removes a CP, and then a Fast Fourier Transform (FFT) unit 340 applies an FFT and provides REs to a subcarrier demapping unit 350, and a controller for reception bandwidth 360 selects the REs used by a transmitter. Next, an Inverse DFT (IDFT) unit 370 applies an IDFT, and a de-multiplexer 380 de-multiplexes a signal received from the IDFT unit 370 in order to generate data bits 390 and CSI bits 395.
Information in an aperiodic CSI report may be determined by a respective aperiodic CSI mode of a UE configured by a NodeB through higher layer signaling. For example, an aperiodic CSI mode may be based on measurements for conveying CQI and PMI for multiple sub-bands, and such a mode may be referred to as mode 2-2, or may be based on measurements for conveying CQI for multiple sub-bands and no PMI, which may be referred to as mode 3-1. An aperiodic CSI mode may be associated with a PDSCH Transmission Mode (TM) of a UE configured by a NodeB through higher layer signaling. For example, for a PDSCH TM of transmission diversity, mode 3-1 may be used, and for a PDSCH TM of spatial multiplexing, mode 2-2 may be used. Similarly, information in a periodic CSI report may be determined by a respective periodic CSI mode of a UE as configured by higher layer signaling and is also associated with a PDSCH TM. For example, for a PDSCH TM using transmission diversity, a periodic CSI report may be based on measurements for conveying wideband CQI, which may be referred to as mode 1-0, or be based on measurements for conveying sub-band CQI and no PMI, which may be referred to as mode 2-0. Additionally, for a PDSCH TM using spatial multiplexing, a periodic CSI report may convey wideband CQI, which may be referred to as mode 1-1, or may convey sub-band CQI and a single PMI, which may be referred to as mode 2-1.
In order to increase transmission data rates to a UE, multiple DL cells may be aggregated and multiple PDSCHs may be respectively transmitted to the UE, and such a process may be referred to as DL Carrier Aggregation (CA). PDSCH transmission parameters in each DL cell may be independent so as to maximize a respective spectral efficiency. DL cells that may convey PDSCHs to a UE in a subframe may be referred to as active DL cells. In order to enable independent link adaptation for each active DL cell, a UE should provide a respective CSI. Similar principles apply for DL Coordinated Multiple Point (COMP) transmission to a UE, wherein multiple Transmission Points (TPs) NodeBs transmit the same data information to a UE.
In a case where a UE transmits a PUCCH using only a single UL cell, wherein the UE may do such because it may not have a UL CA capability or because channel conditions, such as a path-loss, in one UL cell may be more favorable or because of system design simplicity, then transmission of multiple periodic CSI reports for respective multiple DL cells may be problematic if such a transmission relies on a PUCCH that may only have enough payload capacity to reliably convey a periodic CSI for a single DL cell.
The above limitation may be circumvented by a UE transmitting a periodic CSI report for each active DL cell in a different subframe using a PUCCH format conveying a periodic CSI report for a single cell. However, this Time Division Multiplexing (TDM) of periodic CSI reports may need a reporting periodicity that is too large which may lead to spectral efficiency degradation of respective PDSCH transmissions as a DL channel used by a UE may change between successive periodic CSI reports. For example, in a TDD system, a number of UL subframes over 10 total subframes may be small, particularly in order to support high DL data rates.
Alternatively, in order to circumvent the above limitation, an aperiodic CSI may be relied upon. For example, as a UE should convey Transmission Control Protocol (TCP) Acknowledgements (ACKs) in a PUSCH in response to reception of data packets, a NodeB may instruct the UE to also multiplex aperiodic CSI in that PUSCH. This can be done by the inclusion of a 2-bit CSI request field in a DCI format that is for scheduling a PUSCH. An example of the 2-bit CSI request is shown in Table 1. A “00” value indicates no aperiodic CSI multiplexing in a PUSCH, and a “01” value indicates that a UE should multiplex aperiodic CSI only for a DL cell that is linked or paired to an UL cell of the PUSCH transmission of a serving DL cell. Furthermore, a value of “10” or “11” indicates that a UE should multiplex aperiodic CSI for a first set or for a second set, respectively, of DL active cells where the UE is configured the cells in the first set or in the second set by higher layer signaling such as Radio Resource Control (RRC) signaling.
TABLE 1Aperiodic CSI Reportas a Function of CSI request field value.Value of CSI request fieldDescription‘00’No aperiodic CSI report‘01’Aperiodic CSI report for DL cell linked to UL cell(serving cell)‘10’Aperiodic CSI report for a 1st set of cells configured byhigher layers‘11’Aperiodic CSI report for a 2nd set of cells configured byhigher layers
While the approach using the aperiodic CSI may support aperiodic CSI reports for multiple DL cells, it is not an ideal solution for providing timely periodic CSI reports. In general, it may not be possible to support an aperiodic CSI reporting for a CSI measurement process selected from multiple CSI measurement processes as aperiodic CSI reporting is uniquely associated with a predetermined CSI measurement process. For example, there is currently no capability for a CSI request field to indicate aperiodic CSI reporting selected between a conventional aperiodic CSI measurement and a conventional periodic CSI measurement or between a measurement from a first DL RS and a measurement from a second DL RS.
Therefore, there is a need to enable aperiodic CSI reporting for a CSI process selected among multiple CSI processes. Additionally, there is a need to indicate a CSI process, from among multiple CSI processes, to a UE for aperiodic CSI reporting. Furthermore, there is a need to enhance a DCI format capability so as to indicate one or more CSI processes, from among multiple CSI processes, for aperiodic CSI reporting.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.